Three people were injured when two cars were dumped about 16 metres into the Skagit River at about 7 p.m. PT Thursday. All three are expected to recover.

The bridge is about 130 kilometres south of Vancouver — about halfway between the Canadian border and Seattle — and the interstate is a major transportation route and the main link for Canadians driving between Vancouver and Seattle.

Dave Chesson with the Washington Department of Transportation says about 70,000 vehicles move through that part of the I-5 corridor every day.

"The timing couldn't be worse but we're certainly grateful that everybody survived," he said.

"We're working really hard to get the people on the ground to start salvaging this material out of here and looking at the engineering aspects of rebuilding the bridge. In the meantime, our main concern is keeping traffic moving down this corridor and the detours aren't the greatest because they take people through town."

Detours have been set up to try to ease the congestion, but drivers have been told to expect major delays. Officials are urging drivers to avoid the area, especially over the Memorial Day weekend.

Alternate routes and detours

'Exponentially lengthened' commute

Washington State Patrol Sgt. Kirk Rudeen said he was shocked and amazed when he heard the news.

"I've been doing this job for over 24 years now and I've never experienced anything like this."

Rudeen advised commuters to be patient and expect lengthy delays.

"When you’re talking about the major roadway ... that goes through Washington and all the West Coast, you're now taking that freeway traffic, you're putting it onto city streets which are not designed to handle that amount of traffic the commute time is going to be exponentially lengthened," he said.

Washington State Gov. Jay Inslee says he'll work with officials to ensure the bridge is fixed as quickly as possible but it is not clear just how long it could take to rebuild or replace the bridge.

"This is the arterial of commerce and industry for the entire state of Washington," he said.

"It's job No. 1 for all us. I would assume we would have good bipartisan work to make sure this bridge is restored as quickly as humanly possible. We'll find a way to do this."

Officials say engineers and contractors have been on site working on a plan since Thursday evening.

'Big puff of dust'

A tractor-trailer, driven by a 41-year-old from Spruce Grove, Alta., carrying a tall load that hit an upper part of the span is being blamed for taking down the bridge.

"For reasons unknown at this point in time, the semi struck the overhead of the bridge, causing the collapse," said Washington state Patrol Chief John Batiste.

The truck made it off the bridge, and the driver remained at the scene and co-operated with investigators.

Dan Sligh and his wife were in their pickup on Interstate 5 heading to a camping trip when a bridge before them disappeared in a "big puff of dust."

People were perched atop their vehicles in Washington, waiting for rescue, after the bridge over the Skagit River collapsed around 7 p.m. PT Thursday. (King 5 News)

"I hit the brakes and we went off," Sligh told reporters from a hospital, adding he "saw the water approaching ... you hold on as tight as you can."

Sligh, his wife and another man in a different vehicle were dumped into the chilly waters of the Skagit River.

Sligh and his wife were taken to Skagit Valley Hospital with non-life-threatening injuries. The other man was reported in stable condition at United General Hospital in Sedro-Woolley.

Bridge listed as 'functionally obsolete'

Jeremiah Thomas, a volunteer firefighter, said he was driving nearby when he glimpsed something out of the corner of his eye and turned to look.

Mount Vernon, Wash.

"The bridge just went down; it crashed through the water," he said. "It was really surreal."

Crowds of people lined the river to watch the scene unfold.

"It's not something you see every day," said Jimmy O'Connor, the owner of two local pizza restaurants who was driving on another bridge parallel to the one that collapsed. "People were starting to crawl out of their cars."

He said he and his girlfriend were about 366 metres away on the Burlington Bridge when they heard "just a loud bang."

"Then we looked over and saw the bridge was down in the water," he said.

He pulled over and saw three vehicles in the water, including the camping trailer that landed upside-down, he said.

The National Transportation Safety Board is sending an investigative team to the site.

The bridge, built in 1955, has been listed as "functionally obsolete" in a Federal Highway Administration database — a category meaning the design is outdated.